Silicon-based photomultipliers are now being actively developed, and improvements have also been made in systems for detecting extremely weak light using a scintillator and a photomultiplier. Development efforts aiming to achieve better performance are also underway. Above all, silicon photomultipliers (SiPMs), comprising an array of avalanche photodiode (APDs) each connected in series with a quenching resistor, are capable of achieving a high signal-to-noise (SN) ratio and a high dynamic range, and can be driven with a low voltage, and so many applications are anticipated.
An electric charge output from a photomultiplier is usually converted into a voltage by an integrator circuit, sampled and held, and then analog-to-digital (AD) converted. The acquired digital signal is then digital-signal processed, and put into histogram.
X-ray computed tomography (CT) apparatuses using photon counting are estimated to have a count rate of the X-ray incident on the scintillator of about 108 counts per second, and therefore require a readout circuit that is capable of measuring high-speed data over several-hundreds of channels with a high energy resolution simultaneously.
With photon counting, the count rate is mostly lilted by the recovery time of the photomultipliers and the time required in AD conversion. While high performance AD converters have become available, it is still necessary to reduce the recovery time of the photomultipliers, in order to achieve measurements at a high count rate. One possible approach for achieving a shorter recovery time is to reduce the quenching resistance in the photomultiplier (SiPM), by reducing the CR time constant of the capacitor (C) and the resistor (R).
If the quenching resistance becomes too small, however, the photomultiplier might fall incapable of performing the quenching operation. This imposes a limitation in the reduction in the recovery time. Disclosed to alleviate this trade-off is active-quenching in active-quenching, transistors are used instead of the quenching resistors. Active quenching requires each APD to be connected in series with a transistor. To achieve a photoelectric transducer array, it is desirable for an APD to be integrated with a transistor, but high-voltage APDs cannot be easily integrated with low-voltage transistors. Furthermore, the APDs need to be ensured of an aperture ratio.